Recent results demonstrating the potential for Tamoxifen to reduce breast cancer risk have placed a premium on understanding the effects of selective estrogen receptor modifiers (SERM) on the breast. MRI is a new powerful tool capable of providing anatomic and functional information about the breast. This project is aimed at developing anatomic and functional markers of SERM effect on the breast. In this project, breast MRI examinations will be performed on patients that have entered the Tamoxifen trial outlined in project 1. MRI examinations will take place at three institutions close to sites participating in the Tamoxifen trial (UCLA, UPENN, Beth Israel Deaconess Medical Center). Imaging will be performed prior to and at six-month intervals after initiation of treatment, and during the one-year observation time. It is hypothesized that glandular volume and quantitative measures of enhancement will serve as measures of SERM effect. The development of surrogate markers to quantitate SERM effect in the breast is critical to evaluating potential chemo-preventive agents as well as identifying those patients who might best benefit from such treatment. In addition, core biopsy samples will be obtained from areas of the breast that demonstrate enhancement on MRI. In addition, core biopsy samples will be obtained from areas of the breast that demonstrate enhancement on MRI. These areas have a high likelihood of demonstrating a proliferative lesion. Histologic, immunohistochemical and LOH/CGH analysis will be correlated with Tamoxifen effect.